The invention involves a process and a printing machine for determining registration errors due to a change in angular velocity of a print cylinder due to engagement with a printing sheet.
In the printing of sheets of paper or the like by printing machines, the correctly positioned printing of the printed image on the sheet is of considerable importance. This characteristic is identified by the term registration. In order to set the registration, in addition to the printed image, registration marks are used, by which deviations from correctly positioned print are determined and measured by the operator of the printing machine. In a further embodiment of this process, the registration is determined and calculated using sensors in the printing machine. To do this, the sensors detect the registration marks on a printing sheet conveyor belt or the printing sheet and, using the position of the registration marks, determine whether the printing is being done without errors. The process and devices of the state of the art detect and correct errors that occur due to mechanical shifts of the sheet on the conveyor belt or shifts of the conveyor belt. Further, errors occur which are caused by changing of the rotational speed of a print cylinder because of a sheet running between the print cylinder and the conveyor belt. However, the distances covered, according to which the image is applied to the sheet, are defined by a specific time that passes during the movement of the conveyor belt between a sensor signal, or a signal derived from it, and a print gap or nip in a print module, in which the image is applied onto the sheet. As a result, the printed image is applied in a shifted manner onto the sheet in the print modules. This leads to a registration error.
The purpose of the invention is thus to determine the registration errors described above. A further purpose of the invention is to correct the errors that are determined. The purposes of the invention are achieved by application by a print cylinder of at least a first registration mark onto a printing sheet transport conveyor belt prior to a sheet transported on such conveyor belt, and application of at least a second registration mark onto such conveyor belt behind the transported sheet. The first registration mark and second registration mark are detected, and a timing number between the detection of the first registration mark and the second registration mark is calculated. The calculated timing number is then compared to a target value to determine any error due to change in angular velocity of a print cylinder due to engagement with the sheet.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.